When a patient's own heart valve becomes diseased, it can be either repaired or surgically replaced with an artificial valve. There are two basic types of artificial heart valves, mechanical valves and tissue valves. Mechanical valves are made of metal or hard plastic, whereas tissue valves consist of chemically preserved animal tissue, usually extracted from pig (porcine) or cow (bovine). The animal tissue valves are mounted on a supporting frame or "stent". The stent enables the surgeon to insert and mount the valve into the heart with minimal difficulty. The stents themselves are constructed from a polymer material and are covered with DACRON.RTM. cloth that contains a sewing ring. Typically, three stent posts project upwardly from the sewing ring and hold the three valve leaflets suspended in the required geometry.
Animal tissue valves have some inherent advantages over mechanical valves since they do not require the patient to be on chronic anticoagulants. Unfortunately, tissue valves eventually suffer from failure in a manner similar to human heart valves, and therefore need periodic replacement. Currently, the survival rate of bioprosthetic tissue valves is approximately 95% after five years from surgery, but only 40% after fifteen years from surgery.
The failure of these animal tissue valves results from poor mechanical properties. Specifically, the supporting stents are relatively rigid, and cannot mimic the cyclic expansion and contraction of the natural aorta where the valve sits. It is believed that mounting of the valves on such non-physiological stents contributes to mechanical damage caused by repetitive sharp bending at the stent posts. Much of the damage to the valve tissue occurs during valve opening because the supporting stents cannot dilate with the recipient's aorta. Such unnatural behaviour induces sharp curvatures within the leaflets and very high local stresses that damage the leaflet material and ultimately cause it to fail through flexural fatigue.
The inventors have recognized the desirability of providing a bioprosthetic heart valve with a flexible or expansive supporting stent. In Krucinski S., Yesely I., Dokainish M. A., Campbell G. "Numerical Simulation of Leaflet Flexure in Bioprosthetic Valves Mounted on Rigid and Expansile Stents", Journal of Biomechanics (26(8):929-943, 1993), the inventors describe a simulated stent with posts that pivot about their respective bases for reducing compressive commissural stressing in a bioprosthetic heart valve. Also, in pivoting stent posts reduce flexural stresses in pericardial bioprosthetic valves: A numerical analysis, presented at the 1993 conference of ASAIO, New Orleans, May, 1993 the concept of pivoting stents was disclosed.
Another prior art bioprosthetic valve is disclosed in U.S. Pat. No. 5,258,023 (Reger). This valve incorporates a stent comprising a frame which is fully covered by a biochemically inert or physiologically compatible shroud. The frame is in the form of a hollow cylinder of rectangular cross-section which is machined or trimmed to provide a suturing support ring, extended cusp stanchions, and interference free blood flow to the coronary arteries. The frame is joint free but is made slightly deformable to conform to contractile changes of the heart. The Reger Patent discloses that such deformity and expansion permits the frame to compliantly respond to expansion and contraction of the native valve orifice of the beating heart in which the aortic valve is implanted in order to reduce beat-by-beat stress on the aortic valve and anchoring sutures, thereby reducing the likelihood of eventual valve dehiscence.
As indicated above, the inventors have realized that while outward movement of the stent posts reduces local stresses on the leaflet material, it is also extremely important to limit the inward movement of the stent posts in order to reduce compressive flexural stresses on the valve leaflets.
Accordingly, although the provision of flexible stent posts is known in the prior art, no practical heart valve has yet been provided for allowing limited outward movement of the stent posts during systole while also preventing inward movement of the stent posts during diastole so as to reduce commissural stresses in the valve leaflets.
Moreover, the inventors' mathematical modelling (see J. Biomechanics article discussed above) has shown that a compliant supporting frame cannot deform sufficiently outward to enable the required amount of expansion to reduce stresses, and still provide structural rigidity to prevent collapse of the frame inward during valve closure.